This invention generally relates to a speed control system for a manufacturing process and, more particularly, to a speed control system that is particularly well suited to the manufacture of paper boxes.
There are a number of manufacturing processes in which an article that is being processed is transported through and between a number of manufacturing machines. This invention is particularly well suited to processes in which articles are dispensed from a first machine at a variable rate and then are collected in a second machine that transports the articles with an even spatial distribution along a path.
One typical example of such a manufacturing process is found in the paper box manufacturing industry. Stated broadly, in a paper box manufacturing process blanks from a source pass through a machine known as a gluer. The gluer folds the box and applies glue at appropriate locations. The glued and folded box is then dispensed from the gluer into a stacker mechanism.
Typically, the stacker mechanism runs at a speed that allows the folded boxes from the gluer to be stacked in an overlapped relationship. When the boxes emerge from the stacker mechanism, they may merely be collected by an operator and packed in shipping boxes. Alternatively, the boxes may transfer to another machine.
In one particular stacker mechanism, the folded boxes from the gluer are caught between two, slowly moving, conveyer belts. Once caught, the boxes move through the stacker mechanism at a linear rate which is much less than the rate at which the boxes are collected thereby to produce the overlap. The belts in the stacker mechanism compress the boxes, and the compression is useful for a number of different reasons. For example, in some applications the compression of the boxes is used to hold the boxes tightly together in order to assist proper drying of the glue. In other applications, the compression is used, in conjunction with alignment elements, to maintain the boxes in a proper alignment as they pass through the stacker mechanism.
When the compression forces exerted by the stacker mechanism are to be maintained within any particular range, it is important that the boxes be collected along the path of travel through the stacker mechanism with even spacing. Thus, it is desirable to have the stacker mechanism vary the rate of travel through the stacker as a function of the rate at which folded boxes actually emerge from the gluer.
In prior systems, an operator visually monitors the relative spacing and adjusts the stacker mechanism speed to maintain the overlap at an approximately equal spacing. However, the inability to note the occurrence of a pile up and the inherent times required for a human being to react to uneven spacing and to convert that reaction into a physical movement of a control element is such as to introduce long averaging constants into the control loop. As a result, the spacing can still vary over quite a range.
The range is typified in the case of a jam, whereupon the gluer suddenly stops ejecting boxes into the stacker. At this point, the operator must sense that cessation and then stop the stacker mechanism. He must also then start the stacker at some arbitrary interval after the first paper box emerges from the gluer, and must then bring the stacker up to proper operating speed as quickly as possible. In terms of the rate at which boxes are ejected from the gluer and the variations in that rate, the time delays introduced by a human are so great as to make it very difficult to maintain an even spatial distribution of the boxes along the path of travel in the stacker mechanism.